PG::choose_acting: let the pg go down if acting is smaller than min_size

Even if the backfill peer would bring us up to min_size, we can't go
active since build_prior will not consider the interval maybe_went_rw.

Fixes: #9497
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2520 from athanatos/wip-9497

PG::choose_acting: let the pg go down if acting is smaller than min_size

Reviewed-by: Sage Weil <sage@redhat.com>